Fluorescence (FL) techniques have emerged as a mainstream research and development area in science and engineering, particularly in the field of biochemical and biological science. Currently, fluorescent molecules are used as probes for DNA sequencing, fluorescence-activated cell sorting, high throughput screening, and clinical diagnostics.
Fluorescence-based techniques offer high sensitivity, low background noises and broad dynamic ranges. A great number of fluorescent probes have been investigated and are already widely used in biotechnology. Many of them show favorable spectral properties of visible absorption and emission wavelength, high extinction coefficients, and reasonable quantum yields. Upon complexation with proteins and DNA, the fluorescence of the bioprobes can be enhanced/quenched and/or red/blue-shifted, thus enabling visual observation of the biomacromolecular species. Among these, the most useful probes are those that act as “turn-on” sensors, whose fluorescence is activated by the analytes.
Several probes for DNA detection based on fluorescent enhancement have been developed such as phenanthridine and acridine derivatives. Middendorf et al. have reported on ethidium bromide (EB), a well-known phenanthridine derivative, which has already been widely used for DNA-sequencing (U.S. Pat. No. 4,729,947, U.S. Pat. No. 5,346,603, U.S. Pat. No. 6,143,151, U.S. Pat. No. 6,143,153). FL enhancement induced by proteins can be attributed to the interaction with hydrophobic regions of proteins, such as NanoOrange (Molecular Probes, Inc., U.S. Pat. No. 6,818,642) and Nile red (U.S. Pat. No. 6,897,297, U.S. Pat. No. 6,465,208), or reaction with amine groups of proteins in the presence of cyanide or thiols, such as fluorescamine (U.S. Pat. No. 4,203,967) and o-phthaldialdehyde (U.S. Pat. No. 6,969,615, U.S. Pat. No. 6,607,918). The FL of cyanine dyes has been found to increase dramatically upon complexation with DNA and proteins. (U.S. Pat. No. 5,627,027, U.S. Pat. No. 5,410,030). Haugland et al. have reported unsymmetrical cyanine dyes, which possess superior fluorescent characteristics when complexed with nucleic acids (U.S. Pat. No. 5,436,134). The SYPRO® dyes are merocyanine dyes that are essentially non-fluorescent when free in solution but become intensely fluorescent in hydrophobic environments (e.g. SYPRO®Red and SYPRO®Orange dyes of Molecular Probes, Inc., U.S. Pat. No. 6,914,250, U.S. Pat. No. 6,316,267). Water-soluble cyanine dyes, such as Cy3 and Cy5, are commonly used in labeling of DNA or RNA for micorarray (V. R. Iyer et al., Science, 1999, 283, 83). Cy3 and Cy5 have merits of high fluorescence intensity and emission even in solid state, whereas, they are quite unstable and show insufficient detection sensitivity (U.S. Pat. No. 7,015,002).
As described in U.S. Pat. No. 7,109,314, a good fluorescent dye should possess a high fluorescent quantum yield and molecular absorption coefficient, as well as good solubility in aqueous media and stability under ambient conditions. However, most of the dyes discussed above are lipophilic, which are at best, only dispersible in aqueous media. For example, Nile Red, a dye used to stain proteins, should be first dissolved in acetone and then mixed rapidly with water immediately prior to use (J. R. Daban et al, Anal. Biochem, 1991, 199, 169).
Additionally, substantially all of the above-described fluorescent dyes suffer from the problem of aggregation-caused quenching (ACQ). Due to their lipophilic character, these fluorescent dyes are prone to aggregate when dispersed in aqueous media or when bound to biological macromolecules. The close proximity of the chromophores often induces a non-radiative energy transfer mechanism that results in self-quenching of the luminescence. This self-quenching drastically reduces the dyes' fluorescent signal thereby prohibiting their use as efficient bioprobes or biosensors.
Substantial effort has been made to mitigate aggregate formation of these dyes (J. R. Lakowicz, et al. Anal Biochem, 2003, 320, 13). However, only a small number of researchers have focused on the design and synthesis of novel organic molecules or polymers that do not suffer from fluorescent quenching, and moreover, even display enhanced light emission upon aggregation.
Recently, aggregation-induced emission (AIE) has been observed. This phenomena is exactly opposite of ACQ. Some non-emissive dyes can be induced to emit efficiently by the aggregate formation. AIE molecules with high quantum yields ΦF (up to 0.85) and various emission colors (blue, green, yellow and red) have been reported. While the AIE dyes have been used for the construction of efficient optical and photonic devices, the possibility of employing them as bioprobes for detecting biopolymers have been virtually unexplored. Accordingly, there remains a great need for water-soluble “light-up” compounds and probes, for example, for the detection of biomacromolecules such as DNA and proteins.
There is a growing demand for new sensors useful for detecting/sensing biomacromolecules. Sensors based on detecting fluorescence of an analyte such as a biomacromolecule are highly sensitive, thereby lowering detection limits.
Many known fluorescent materials accomplish the detection of saccharides by the competing intramolecular interaction of an amine functionality with a boronic acid pendant. Less effort has been spent on the detection of other biological compounds. Furthermore, vapor-sensing compounds and devices are often manufactured from the expensive platinum salts and complexes and/or in combination with palladium. They are based mainly on a color shift from dark-red to light-red, making it difficult to visually sense the color shift. Sensors exhibiting an on-off change in their luminescent color rather than a color shift will be thus not only advantageous but also more sensitive. To applicants' knowledge, the only known “on-off” example was shown by Kato (U.S. Pat. No. 6,822,096), who utilized the luminescence change from the invisible near-infrared to the visible red of binuclear platinum (II) complexes. However, these complexes only shift the emitted wavelength out of the visible spectrum.
Fluorescent materials, including inorganic semiconductor quantum dots, organic and metallorganic dyes, dye-doped silica or polymer particles, have currently attracted great attention in a wide variety of high-technology applications such as high-throughput screening, ultra-sensitive assays, optoelectronics, and living cell imaging. Colloidal quantum dots (hundreds to thousands of atoms) are traditionally made from crystals of IIA-VIA or IIIB-VB elements (PbS, CdSe, etc.) or other semiconductors. The heavy metals therein are intrinsically toxic to the researchers and the experimental systems (e.g., living cells), as well as generating a toxic waste stream into the environment. Organic and metallorganic dyes generally consist of π-conjugated ring structures such as xanthenes, pyrenes or cyanines, with emissions across the spectrum from UV to the near infrared (˜300-900 nm) and may be fine tuned to particular wavelengths or applications by changing the chemistry of their substituent groups. The size of individual dye molecules is very small (˜1 nm), which causes non-specific labeling and high background signals as dyes diffuse away from their intended targets. Spectrally, organic dyes tend to have fairly wide absorption and emission spectra (FWHM ˜50 nm), which can lead to spectral overlap and re-absorption when using multiple dye species simultaneously. In normal use, dye molecules are exposed to a variety of harsh environments and often suffer from photobleaching and quenching due to the interactions with solvent molecules and reactive species such as oxygen or ions dissolved in solution.
In order to create more robust emitters with enhanced brightness and stability, researchers have developed composite nano- and micro-particles consisting of dye molecules and silica or polymer matrix. Thus the encapsulated dye molecules can be protected from external perturbations, with reducing stochastic blinking, photobleaching, and quenching. Dye-loaded polymer particles are superior to their silica counterpart in terms of the versatile chemical compositions, tunable surface chemistry suited for biocompatibility and bioconjugation, facile preparation, and easy control of the particle size and size distribution.
Gao et al. have incorporated pyrene dyes into polystyrene particles using a normal microemulsion approach, leading to a 40-fold increase in emission intensity with respect to the pure dye at the identical concentration (H. Gao et al., Colloid Polym. Sci. 2002, 280, 653). Dinsmore et al. swelled poly(methyl methacrylate) particles and absorbed a rhodamine dye into them for usage in a confocal microscopic study of colloidal dispersions (A. D. Dinsmore et al., Appl. Opt. 2001, 40, 4152). U.S. Pat. No. 5,716,855 disclosed fluorescent particles containing anthracene- or naphthacene-derivatived dyes aiming to the application as biological markers.
Up to now, most of the organic dyes commercially available, including the above mentioned dyes as well as ethidium bromide (U.S. Pat. No. 4,729,947, U.S. Pat. No. 5,346,603, U.S. Pat. No. 6,143,151, and U.S. Pat. No. 6,143,153), Nile red (U.S. Pat. No. 6,897,297 and U.S. Pat. No. 6,465,208), fluorescamine (U.S. Pat. No. 4,203,967), o-phthaldialdehyde (U.S. Pat. No. 6,969,615 and U.S. Pat. No. 6,607,918), Cyanine dyes (U.S. Pat. No. 5,627,027 and U.S. Pat. No. 5,410,030), etc. are emissive only in their solution state, whereas emission is quenched in aggregation states (e.g., high dye concentration state, film state, solid state, etc.). This is attributed to the mechanism of nonradiative energy transfer between the closely packed chromophores, thus resulting in self-quenching of the fluorescence. Thus, the loading concentration of dyes in the polymer particles cannot be sufficiently high and accordingly the intensity of fluorescence is considerably limited.
With respect to the polymers for dye encapsulation, the currently available species are mainly hydrophobic polystyrene and less hydrophobic poly(methyl methacrylate), as mentioned hereinabove. The hydrophobic nature of these particles commonly leads to clustering and non-specific binding of biological materials, which considerably limits their application in aqueous environment of biology and other fields. Additionally, these particles are prepared and dispersed in organic solvent. For example, Hu et al. prepared poly(methyl methacrylate) fluorescent particles through dispersion polymerization in the mixture of hexane and ethanol (H. Hu et al., Langmuir 2004, 20, 7436). The solvent-dispersible polymer particles are difficult to disperse stably in aqueous media.
The presently described series of linear and cyclic π-conjugated organic compounds (hereinafter polyenes) have been designed and synthesized with different chromophores including tetraphenylethylene, siloles, fulvene, butadienes, and 4H-pyrans. The emission color of these new polyenes ranges from blue to red arising from the different chromophoric structures. Their fluorescent behavior features the aggregation-induced emission (AIE) phenomenon, which turns the dyes from faint-emitters when molecularly dissolved into strong luminophors when aggregated or in the solid state. All these features make the presently described AIE-active molecules excellent candidates for use as bioprobes for DNA detection, G-quadruplex identification and potassium-ion sensing as well as in polymeric particles, sensors and detection devices. In addition, the AIE-active-molecules can be used to study conformational structures, folding processes and as fluorescent markers to visualize DNA bands in assays.